Carrier catalyst, process for the production therefor for the preparation of vinyl acetate

ABSTRACT

The invention relates to Pd/K/Au, Pd/K/Ba or Pd/K/Cd supported catalysts built up in the form of an outer layer, the production thereof and the use thereof for preparing vinylacetate from ethylene, acetic acid and oxygen in the gas phase. The specified catalysts are produced by impregnating the support particles with a solution of salts of the corresponding elements and then drying them, with the dynamic viscosity of the solution being at least 0.003 Pa.s and the solution volume in impregnation being more than 80% of the pore volume of the support particles and the duration of the impregnation and also the time until commencement of the drying being selected so as to be sufficiently short for, after completion of the drying, the specified metal salts to be present in an outer layer of from 5% to 80% of the pore volume of the support particles.

This is a continuation of Ser. No. 443,338 filed May 17, 1995, nowabandoned, which is a divisional of Ser. No. 08/276,438 filed Jul. 18,1994, now U.S. Pat. No. 5,571,771.

It is known that vinyl acetate can be prepared in the gas phase fromethylene, acetic acid and oxygen; the supported catalysts used for thissynthesis comprise palladium and an alkali metal element, preferablypotassium. Further additives used are cadmium, gold or barium.

In the Pd/K/Au catalysts both noble metals are generally applied in theform of an impregnated layer on the support; they are produced byimpregnation and subsequent precipitation of the metal salts by means ofalkaline compounds (U.S. Pat. Nos. 4,048,096, 3,775,342).

In Pd/K/Ba catalysts the metal salts are applied by impregnation,spraying on, vapor deposition, dipping or precipitation (EP-A-0 519436). The same methods are known for Pd/K/Cd catalysts (U.S. Pat. Nos.4,902,823; 3,393,199, 4,668,819). Furthermore, the production of aPd/K/Au or Pd/K/Cd surface impregnated catalyst is known, with aspecific support material being washed with an acid prior toimpregnation and being treated with a base after impregnation (EP-A-0519 435).

The German Patent Application P 42 11 780.1 which corresponds to U.S.Pat. No. 5,422,329 describes Pd/K/Au, Pd/K/Ba or Pd/K/Cd catalysts builtup in the form of an outer layer by atomizing a solution of thecorresponding metal salts by means of ultrasound and then, in a limitedamount and over a limited time, applying this to the support particlesand commencing the drying thereof in such a way that the catalyticallyactive metal salts cannot penetrate into the core of the supportparticles, but only into an outer part having a greater or lesserthickness, the impregnated layer.

It has now been found that surface impregnated catalysts containing thespecified elements are obtained much more simply by impregnating thesupport particles with viscous solution of corresponding metal salts,instead of applying a solution after atomization into very fine dropletsby means of ultrasound. In this impregnation, the amount of solution hasno upper limit, but the impregnation has to be carried out sufficientlyquickly and the drying has to be commenced sufficiently quickly, similarto the case of the solution atomized by ultrasound in accordance with P42 11 780.1 which corresponds to U.S. Pat. No. 5,422,329.

The invention provides a process for producing a surface impregnatedcatalyst comprising palladium, potassium and cadmium on porous supportparticles, which comprises impregnating the support particles once or aplurality of times with at least one solution of at least one salt ofeach of the three elements and drying the support particles after eachimpregnation, with the dynamic viscosity of the solution being at least0.003 Pa.s and the solution volume in each impregnation being more than80% of the pore volume of the support particles, and with the durationof each impregnation and also the time until commencement of the dryingfollowing this impregnation being selected so as to be sufficientlyshort for, after completion of the last drying, the specified metalsalts to be present in an outer layer of from 5% to 80% of the porevolume of the support particles.

The invention also provides a surface impregnated catalyst produced inthis manner, and the use thereof for preparing vinyl acetate fromethylene, acetic acid and oxygen in the gas phase.

In this drying, the amount of solution has no upper limit; a higherexcess of solution does no harm, but is superfluous.

The invention further provides a process for producing a surfaceimpregnated catalyst comprising palladium, potassium and barium onporous support particles, which comprises impregnating the supportparticles once or a plurality of times with at least one solution of atleast one salt of each of the three elements and drying the supportparticles after each impregnation, with the dynamic viscosity of thesolution being at least 0.003 Pa.s and the solution volume in eachimpregnation being more than 80% of the pore volume of the supportparticles, and with the duration of each impregnation and also the timeuntil commencement of the drying following this impregnation beingselected so as to be sufficiently short for, after completion of thelast drying, the specified metal salts to be present in an outer layerof from 5% to 80% of the pore volume of the support particles.

The invention also provides a surface impregnated catalyst produced inthis manner, and the use thereof for preparing vinyl acetate fromethylene, acetic acid and oxygen in the gas phase.

Here too, the amount of solution has no upper limit.

The invention further provides a process for producing a surfaceimpregnated catalyst comprising palladium, potassium and gold on poroussupport particles, which comprises impregnating the support particlesonce or a plurality of times with at least one solution of at least onesalt of each of the three elements and drying the support particlesafter each impregnation, with the dynamic viscosity of the solutionbeing at least 0.003 Pa.s and the solution volume in each impregnationbeing more than 80% of the pore volume of the support particles, andwith the duration of each impregnation and also the time untilcommencement of the drying following this impregnation being selected soas to be sufficiently short for, after completion of the last drying,the specified metal salts to be present in an outer layer of from 5% to80% of the pore volume of the support particles.

The invention also provides a surface impregnated catalyst produced inthis manner, and the use thereof for preparing vinyl acetate fromethylene, acetic acid and oxygen in the gas phase.

Here too, the amount of solution has no upper limit.

Supports used are inert materials such as silicon dioxide, aluminumoxide or mixtures of these oxides in the form of spheres, pellets,rings, stars or other shaped bodies; the diameter, or the length andthickness, of the support particles is generally from 3 to 9 mm.

The surface area of the supports is, measured by the BET method,generally from 50 to 250 m² /g; the pore volume is generally from 0.4 to1.2 ml/g.

The metal contents of the finished catalysts are as follows:

The palladium content of the Pd/K/Cd and the Pd/K/Ba catalysts is ingeneral from 0.6 to 3.5% by weight, preferably from 0.8 to 3.0% byweight, in particular from 1.0 to 2.5% by weight. The palladium contentof the Pd/K/Au catalysts is in general from 0.5 to 2.0% by weight,preferably from 0.6 to 1.5% by weight.

The potassium content of all three types of catalyst is in general from0.5 to 4.0% by weight, preferably from 1.5 to 3.0% by weight.

The cadmium content of the Pd/K/Cd catalysts is in general from 0.1 to2.5% by weight, preferably from 0.4 to 2.0% by weight.

The barium content of the Pd/K/Ba catalysts is in general from 0.1 to2.0% by weight, preferably from 0.2 to 1.0% by weight.

The gold content of the Pd/K/Au catalysts is in general from 0.2 to 1.0%by weight, preferably from 0.3 to 0.8% by weight.

Suitable salts are all salts of palladium, cadmium, barium, gold andpotassium which are soluble and contain no constituents, e.g. sulfur,which poison the catalyst; preference is given to the acetates and thechlorides. However, in the case of the chlorides, it must be ensuredthat the chloride ions are removed before the catalyst is used. This isachieved by washing the doped support, for example with water, afterpalladium, and, if present, gold have been converted into an insolubleform, for instance by reduction and/or precipitation with hydroxides.

Suitable solvents are all compounds in which the selected salts aresoluble and which after impregnation can easily be removed again bydrying. Suitable solvents for the acetates are, in particularunsubstituted carboxylic acids having from 2 to 10 carbon atoms, such asacetic acid, propionic acid, n- and iso-butyric acid and the variousvaleric acids. Owing to its physical properties and also for economicreasons, acetic acid is the preferred carboxylic acid. For thechlorides, water is particularly suitable. The additional use of afurther solvent is advantageous if the salts are not sufficientlysoluble in acetic acid or in water. Suitable additional solvents arethose which are inert and miscible with acetic acid or water. Additivesfor acetic acid which may be mentioned are ketones such as acetone andacetylacetone, furthermore ethers such as tetrahydrofuran or dioxane, orelse hydrocarbons such as benzene.

At least one salt of each of the three elements to be applied to thesupport particles (Pd/K/Cd, Pd/K/Ba, Pd/K/Au) has to be applied. Aplurality of salts of one element can be applied, but in general exactlyone salt of each of the three elements is applied.

The three elements to be applied in each case can be individuallyapplied in the form of salt solutions, or otherwise in any combinations.Use is preferably made of a single solution which contains all threeelements to be applied in the form of salts. Particular preference isgiven to the use of a single solution which contains exactly one salt ofeach of the three elements to be applied.

In general reference is made hereinafter to "the solution of the salts"the same applies analogously to the case in which use is made in orderof a plurality of solutions which contain only part of the salts to beapplied, with the individual parts adding up to the total amount of thesalts which are to be applied to the support.

The solution of the salts is applied to the support particles byimpregnating these once or a plurality of times with this solution, withthe solution volume in each impregnation being more than 80% of the porevolume of the support particles. Subsequent to impregnation, the supportparticles are dried; in the case of a plurality of impregnations, dryingis carried out after each impregnation.

The duration of each impregnation and also the time until commencementof the drying following this impregnation has to be selected so as to besufficiently short for, after completion of the last drying, thespecified metal salts to be present in an outer layer of from 5% to 80%of the pore volume of the support particles.

The dynamic viscosity of the solution is at least 0.003 Pa.s, the upperlimit is determined by the solubility of the salts used in the solventselected. Preferably, the dynamic viscosity is from 0.005 to 0.009 Pa.s,in particular from 0.006 to 0.008 Pa.s.

If the solution volume selected is not sufficiently high for all supportparticles to be completely within the solution during impregnation, thesupport particles have to be intimately mixed during impregnation, forexample in a rotating or agitated flask or a mixing drum, to ensure auniform impregnated layer thickness in all support particles. Therotation rate or intensity of agitation has to, on the one hand, be highenough to ensure good mixing, but, on the other hand, must not be sohigh that the support material is significantly abraded.

A suitable method of determining the desired distribution of theimpregnated layer thicknesses comprises cutting open a representativenumber of support particles and measuring impregnated layer thicknessesunder a microscope. Here, preferably less than 5% of the particlesshould have an impregnated layer thickness which deviates by more than15% from the average.

The solution of the salts should be at a temperature which is highenough to prevent precipitation of the salts during application to thesupport. However, the temperature should generally not be significantlyabove 70° C. to prevent excessive evaporation of the solvent.

During the drying of the support impregnated with the solution of theactive catalyst components, it is advisable to match the temperature tothe type of metal salts used. In the case of the acetates, which arefrequently used for the production of Pd/K/Cd or Pd/K/Ba catalysts,drying is preferably carried out at reduced pressure. The temperaturehere should generally be from 50° to 80° C., preferably from 50° to 70°C. Furthermore, it is generally advisable to carry out the drying in aninert gas stream, for example in a nitrogen or carbon dioxide stream. Inthe case of the Pd/K/Au catalysts, which are generally impregnated withthe corresponding chlorides, the drying can, in contrast, be carried outin a hot air stream at from 100° to 150° C. The residual solvent contentafter drying should preferably be less than 6% by weight for all threetypes of catalyst.

If a reduction of the palladium salt and, if applicable, of the goldsalt is to be carried out, which is sometimes useful, this can becarried out using a gaseous reducing agent. The reduction temperature isgenerally between 40° and 260° C., preferably between 70° and 200° C. Ingeneral it is advantageous to carry out the reduction using a reducingagent diluted with inert gas, which contains from 0.01 to 50% by volume,preferably from 0.5 to 20% by volume, of reducing agent. The inert gasused can be, for example, nitrogen, carbon dioxide or a noble gas.Suitable reducing agents are, for example, hydrogen, methanol,formaldehyde, ethylene, propylene, isobutylene, butylene or otherolefins. The amount of the reducing agent depends on the amount ofpalladium and, if applicable, on the amount of gold; the reductionequivalent should be at least 1 to 1.5 times the oxidation equivalent,but larger amounts of reducing agent do no harm. Such a reduction iscarried out subsequently to drying.

The preparation of vinyl acetate is generally carried out by passingacetic acid, ethylene and oxygen or oxygen-containing gases over thefinished catalyst at temperatures of from 100° to 220° C., preferablyfrom 120° to 200° C., and at pressures of from 1 to 25 bar, preferablyfrom 1 to 20 bar, with unreacted components being able to berecirculated. The oxygen concentration is advantageously kept below 10%by volume (based on the gas mixture free of acetic acid). However,dilution with inert gases such as nitrogen or carbon dioxide issometimes also advantageous. Carbon dioxide is particularly suitable forthe dilution, since it is formed in small amounts during the reaction.

The catalysts of the invention make it possible for the process to becarried out more selectively than when using catalysts in which thesupport particles are impregnated right into the core ("impregnatedthrough"), or make possible an expansion of capacity. To achieveexpansion of capacity, it is possible to keep the reaction conditions(e.g. pressure, temperature, throughput, oxygen concentration) unchangedin comparison with the known catalysts, and to prepare more vinylacetate per reactor volume and time. This makes the workup of the crudevinyl acetate obtained easier, since the vinyl acetate content in theexit gas from the reactor is higher, which leads further to a saving ofenergy in the workup section. A suitable workup is described, forexample, in U.S. Pat. No. 5,066,365.

If, in contrast, the plant capacity is kept constant, the reactiontemperature can be lowered and thereby the reaction can be carried outmore selectively at the same total output, which saves startingmaterial. Here the amount of carbon dioxide, which is formed asby-product and therefore has to be removed, and the loss of entrainedethylene associated with this removal are also smaller. In addition,this mode of operation prolongs the operating life of the catalyst.

The following examples illustrate the invention.

The catalyst support used was SiO₂ in the form of pellets having adiameter and a length of 6 mm in each case. The pellets were pressedfrom ®Aerosil powder with the aid of magnesium stearate as binder inaccordance with DE-A 3 912 504. The surface area of the support was 120m² /g, its pore volume was 0.784 ml/g and its bulk density was 500 g/l.The pore volume of 1 l of support was 392 ml.

COMPARATIVE EXAMPLE 1a

1 l of silica support was impregnated at 60° C. with a solution of 24.3g of palladium acetate, 21.3 g of cadmium acetate and 23.8 g ofpotassium acetate in 392 ml of glacial acetic acid (solution volume=100%of the pore volume of the support). The material was subsequently driedin a drying cabinet at 200 mbar under nitrogen to a residual acetic acidcontent of 6% by weight; the drying temperature was 65° C. The finishedcatalyst contained 2.3% by weight of Pd, 1.8% by weight of Cd and 1.9%by weight of K.

A reaction tube having an internal diameter of 8 mm and a length of 1.5m were charged with 50 ml of this catalyst. The gas to be reacted wasthen passed over the catalyst at a pressure of 8 bar (reactor inlet) anda catalyst temperature of 150° C. This gas comprised 27% by volume ofethylene, 55% by volume of nitrogen, 12% by volume of acetic acid and 6%by volume of oxygen. The results are shown in the table.

COMPARATIVE EXAMPLE 1b

25.3 g of palladium acetate, 25 g of cadmium acetate and 25.3 g ofpotassium acetate were dissolved at 65° C. in 137.2 ml of acetic acid(solution volume=35% of the pore volume) and the highly viscous solutionwas placed in a reservoir preheated to 65° C. 1 l of catalyst supportwas likewise heated to 65° C. in a mixing drum able to be temperaturecontrolled and was mixed at a rotation rate of 150 rpm. The impregnationsolution was applied to the catalyst support by means of an ultrasoundatomizer (100 kHz) over a period of 1 hour.

The material was subsequently dried as in Comparative Example 1a. Thefinished catalyst contained 2.3% by weight of Pd, 1.8% by weight of Cdand 1.9% by weight of K. The impregnated layer thickness was 0.8 mm.

The catalyst was tested as in Comparative Example 1a and the results areshown in the table.

COMPARATIVE EXAMPLE 2a

The catalyst was produced as in Comparative Example 1a, except that now4.0 g of barium acetate were applied in place of cadmium acetate. Thefinished catalyst contained 2.3% by weight of Pd, 0.4% by weight of Baand 1.9% by weight of K.

The catalyst was tested as in Comparative Example 1a and the results areshown in the table.

COMPARATIVE EXAMPLE 2b

The catalyst was produced as in Comparative Example 1b, except that now4.3 g of barium acetate were used in place of cadmium acetate. Thefinished catalyst contained 2.3% by weight of Pd, 0.4% by weight of Baand 1.9% by weight of K, the impregnated layer thickness was 0.8 mm.

The catalyst was tested as in Comparative Example 1a and the results areshown in the table.

COMPARATIVE EXAMPLE 3a

1 l of silica support was, in accordance with EP-A-0 519 435, washedwith 10% strength hydrochloric acid and then with water to remove thebinder interfering with the formation of the impregnated layer anddried. The support was subsequently impregnated with a solution of 13.8g of sodium chloropalladate and 4.0 g of tetrachloroauric acid in 392 mlof water. After drying with hot air at 150° C., 5.5 g of NaOH dissolvedin 392 ml of water were added, to produce an impregnated layer byprecipitation of palladium and gold. The material was subsequentlystirred for 6 hours and allowed to stand for 16 hours at roomtemperature. After the support had been washed free of chloride withwater and dried with hot air at 150° C., 35.1 g of potassium acetate in392 ml of water were applied. After drying with hot air at 150° C. thecatalyst contained 1.0% by weight of Pd, 0.4% by weight of Au and 2.8%by weight of K. The thickness of the impregnated layer produced by thetreatment with sodium hydroxide solution was from 1.3 to 1.6 mm. Thecatalyst was tested in a Berty reactor at 152° C. using a gas mixture of8% by volume of O₂, 37.5% by volume of C₂ H₄, 15.7% by volume of HOAcand 38.8% by volume of N₂ ; the results are shown in the table.

COMPARATIVE EXAMPLE 3b

13.8 g of sodium chloropalladate and 4.0 g of tetrachloroauric acid weredissolved in 78.4 ml of water (solution volume=20% of the pore volume).The solution was applied at room temperature to 1 l of catalyst supportby means of an ultrasound atomizer (100 kHz) over a period of 1 hour;the material was subsequently dried in a hot air stream at 150° C. Toprecipitate palladium and gold, a solution of 5.5 g of NaOH in 78.4 mlof water was then applied to the impregnated support using theultrasound atomizer. It was then, in the same way as in ComparativeExample 3a, washed free of chloride and dried. It was subsequentlyreduced with H₂, impregnated with 35.1 g of potassium acetate in 392 mlof water and dried using hot air at 150° C.

The finished catalyst contained 1.0% by weight of Pd, 0.4% by weight ofAu and 2.8% by weight of K; the impregnated layer thickness was 0.7 mm.

The catalyst was tested as in Comparative Example 3a and the results areshown in the table.

EXAMPLE 1

759 g of palladium acetate, 750 g of cadmium acetate and 759 g ofpotassium acetate were dissolved at 65° C. in 3.9 l of acetic acid(solution volume about 10 times the pore volume) and the highly viscoussolution (7 mPa.s) was placed in a reservoir preheated to 65° C. 1 l ofthe catalyst support was likewise heated to 65° C. and combined with thewhole of the impregnation solution. After 3 minutes, the supportparticles were separated from the solution by means of a plastic screen.

Subsequently, drying was commenced within 3 minutes, which drying wasotherwise carried out as in Comparative Example 1a. The finishedcatalyst contained 2.3% by weight of Pd, 1.8% by weight of Cd and 1.9%by weight of K. The impregnated layer thickness was 0.8 mm.

The catalyst was tested as in Comparative Example 1a and the results areshown in the table.

EXAMPLE 2

The catalyst was produced as in Example 1, except that now 129 g ofbarium acetate were used in place of cadmium acetate. The finishedcatalyst contained 2.3% by weight of Pd, 0.4% by weight of Ba and 1.9%by weight of K, the impregnated layer thickness was 0.8 mm.

The catalyst was tested as in Comparative Example 1a and the results areshown in the table.

EXAMPLE 3

690 g of sodium chloropalladate and 200 g of tetrachloroauric acid weredissolved in 3.92 l of water (solution volume about 10 times the porevolume). 1 l of catalyst support was impregnated, as described inExample 1, with this solution at room temperature for 3 minutes;subsequently drying in a hot air stream at 150° C. was commenced within2 minutes. To precipitate palladium and gold, the impregnated supportparticles were then impregnated for 3 minutes with a solution of 275 gof NaOH in 3.92 l of water. They were then, in the same way as inComparative Example 3a, washed free of chloride and dried. They weresubsequently reduced with H₂, impregnated with 1755 g of potassiumacetate in 3.92 l of water for 3 minutes, and drying in a hot air streamat 150° C. was commenced within 2 minutes.

The finished catalyst contained 1.0% by weight of Pd, 0.4% by weight ofAu and 2.8% by weight of K; the impregnated layer thickness was 0.7 mm.

The catalyst was tested as in Comparative Example 3a and the results areshown in the table.

                  TABLE                                                           ______________________________________                                                                Vinylacetate content                                                  Spec.   % by weight  Selec-                                             Output                                                                              output  in the condensed                                                                           tivity                                              g/1 h!                                                                             (*)     reactor exit gas                                                                            %!                                      ______________________________________                                        Comp. Example 1a                                                                          813     70.7    25.7       94.3                                   (Pd/K/Cd □)                                                        Comp. Example 1b                                                                          915     79.6    33.0       96.3                                   (Pd/K/Cd ∘)                                                       Comp. Example 2a                                                                          827     71.9    25.9       92.8                                   (Pd/K/Ba □)                                                        Comp. Example 2b                                                                          917     79.7    33.1       95.7                                   (Pd/K/Ba ∘)                                                       Comp. Example 3a                                                                          710     142.0   22.6       89.3                                   (Pd/K/Au #)                                                                   Comp. Example 3b                                                                          740     148.0   24.2       90.0                                   (Pd/K/Au ∘)                                                       Example 1   913     79.4    32.9       96.0                                   (Pd/K/Cd)                                                                     Example 2   912     79.3    32.9       95.9                                   (Pd/K/Ba)                                                                     Example 3   743     148.6   24.3       90.8                                   (Pd/K/Au)                                                                     ______________________________________                                         * Gram of vinyl acetate per gram of palladium per hour                        □ Catalyst impregnated right through to the core of the suppor     particles                                                                     ∘ Surface impregnated catalyst produced by means of ultrasoun     spraying                                                                      # Surface impregnated catalyst in accordance with EPA-0 519 435               (precipitation with base)                                                

Thus, essentially the same results are achieved in the Examples of theinvention 1 to 3 as in the Comparative Examples 1b, 2b and 3b in whichthe catalysts were produced by ultrasound spraying. Surprisingly, it istherefore not at all necessary for the production of a very highperformance surface impregnated catalyst to divide the impregnationsolution into extremely fine droplets by means of the complicated use ofultrasound, but the impregnation of the invention is sufficient.

As the Comparative Examples 1a, 2a and 3a show, the performance data ofimpregnated-through catalysts or a surface impregnated catalyst inaccordance with EP-A-0 519 435 are significantly worse.

What is claimed is:
 1. A process for preparing vinyl acetate in the gasphase from ethylene, acetic acid and oxygen or oxygen-containing gasesin the presence of a surface impregnated catalyst comprising palladium,potassium and cadmium on porous support particles, wherein the supportparticles are impregnated once or a plurality of times with at least onesolution of at least one salt of each of the three elements, and saidsolutions are not atomized by ultrasound prior to the impregnation andthe support particles are immediately dried after each impregnation,with the dynamic viscosity of the solution being at least 0.003 Pa.s andthe solution volume in each impregnation being from more than 80% of thepore volume of the support particles, and the duration of eachimpregnation and also the time until commencement of the dryingfollowing this impregnation being selected so as to be sufficientlyshort for, after completion of the last drying, the specified metalsalts to be present in an outer layer of from 5% to 80% of the porevolume of the support particles.
 2. The process as claimed in claim 1,wherein the dynamic viscosity of the solution is from 0.005 to 0.009Pa.s.
 3. A process for preparing vinyl acetate in the gas phase fromethylene, acetic acid and oxygen or oxygen-containing gases in thepresence of a surface impregnated catalyst comprising palladium,potassium and barium on porous support particles, wherein the supportparticles are impregnated once or a plurality of times with at least onesolution of at least one salt of each of the three elements, and saidsolutions are atomized by ultrasound prior to the impregnation and thesupport particles are immediately dried after each impregnation, withthe dynamic viscosity of the solution being at least 0.003 Pa.s and thesolution volume in each impregnation being from more than 80% of thepore volume of the support particles and the duration of eachimpregnation and also the time until commencement of the dryingfollowing this impregnation being selected so as to be sufficientlyshort for, after completion of the last drying, the specified metalsalts to be present in an outer layer of from 5% to 80% of the porevolume of the support particles.
 4. The process as claimed in claim 3,wherein the dynamic viscosity of the solution is from 0.005 to 0.009Pa.s.
 5. A process for preparing vinyl acetate in the gas phase fromethylene, acetic acid and oxygen or oxygen-containing gases in thepresence of a surface impregnated catalyst comprising palladium,potassium and gold on porous support particles, wherein the supportparticles are impregnated once or a plurality of times with at least onesolution of at least one salt of each of the three elements, and saidsolutions are not atomized by ultrasound prior to the impregnation andthe support particles are immediately dried after each impregnation,with the dynamic viscosity of the solution being at least 0.003 Pa.s andthe solution volume in each impregnation being from more than 80% of thepore volume of the support particles and the duration of eachimpregnation and also the time until commencement of the dryingfollowing this impregnation being selected so as to be sufficientlyshort for, after completion of the last drying, the specified metalsalts to be present in an outer layer of from 5% to 80% of the porevolume of the support particles.
 6. The process as claimed in claim 5,wherein the dynamic viscosity of the solution is from 0.005 to 0.009Pa.s.
 7. The process as claimed in claim 1, wherein said dynamicviscosity of the solution is from 0.006 to 0.008 Pa.s.
 8. The process asclaimed in claim 3, wherein said dynamic viscosity of the solution isfrom 0.006 to 0.008 Pa.s.
 9. The process as claimed in claim 5, whereinsaid dynamic viscosity of the solution is from 0.006 to 0.008 Pa.s.